Blue/green/UV high brightness light emitting diodes (LEDs) are the next generation of solid state LED emitters. They are penetrating into a broad range of applications such as traffic signaling, medical uses, outdoor full motion LED video signage, and stage and building lighting. A combination of blue or UV LEDs and phosphors produces white LEDs, which will take on a whole new meaning for many far reaching applications such as: general illumination, outdoor signal, automobile lighting with built in safety features, and many more. The public has realized the benefits that can be obtained with solid state blue/green/UV/white LEDs and to-date, a large volume of research on GaN based, ZnO based and related materials has been conducted. GaN-based LEDs are manufactured with mass production in the US, Europe, and several Asian countries.
Often an LED includes a P-doped semiconductor layer approximate an N-doped semiconductor layer, often with quantum wells which can be considered between the P-doped layer and the N-doped layer. Injecting current into the device, such that the P-N junction is forward-biased, causes the device to emit light.
FIGS. 5A and 5B are top views of typical GaN-based LED structures with a pair of P and N electrodes that are circular, square or rectangular and located at diagonally opposite corners of the LED chip. The electrodes are generally metal alloys with various thicknesses suitable for wire bonding connections to the device. As illustrated, FIG. 5A includes square contacts 517, 519 and FIG. 5B includes circular, or dot, electrodes 531, 533. Assuming in FIGS. 5A and 5B that the P-doped material is over the N-doped material, the P-doped material forms a mesa 511, with a portion of the top of N-doped material exposed about one corner 515 of the LED, which may be accomplished by way of etching or the like. The N electrode is on the exposed portion of the N-doped material. The P electrode is approximate an opposing corner of the LED, and is on the P-doped material or a current spreading layer deposited on top of the P-doped material, or a combination of the two.
These electrodes may be considered dot-like current electrodes, or simply dot-like electrodes. The primary difficulty of dot-like electrodes is current crowding, which tends to occur near the electrical contact of the LED chip because of the tendency of charge carriers to travel through a path of least resistance. As a result, current does not spread evenly over the entire structure of the LED chip, but segregates near the contact electrode. FIG. 6 shows a picture of current crowding near N electrode area of a GaN blue LED chip at an injection current of 20 mA. The current crowding problem can be partially remedied by increasing the thickness of a current spreading layer. However, the thicker the current spreading layer, the more light is absorbed. The current crowding effect can also be reduced by increasing the thickness of the N-type layer. As a drawback, the thicker the N-type layer, the greater the possibility of increasing defects in the crystal film, which could lower the quality and performance of the LED.